Objective  To analyze the clinical characteristics of COVID-19 cases whose serological antibody IgM and IgG were both negative, and try to find the causes.

  Methods  Clinical data of COVID-19 cases in Guanggu Branch of Maternity and Child Health Care Hospital of Hubei Province (a COVID-19 designated hospital) who were discharged or deceased before March 23, 2020 were collected, and clinical characteristics between negative IgM and IgG cases (double negative group) and positive IgM and IgG cases (double positive group) were compared and analyzed.

  Results  There were 77 cases in the double negative group, with 34 males and 43 females and an average age of 62.34±19.69 years. Another 100 cases were randomly selected from the double positive group, with 39 males and 61 females and an average age of 59.72±15.48 years. There was no statistically significant difference in sex and age between the two groups (P > 0.05). More asymptomatic cases were found in the double negative group than in the double positive group 35.06% (27/77) vs 11.00% (11/100), P=0.000, and more cases showed a normal chest CT imaging in the double negative group than in the double positive group 18.18% (14/77) vs 1.00% (1/100), P=0.000. The proportion of severe and critically severe cases in the double negative group was 16.88% (13/77), and 15.00% (15/100) in the double positive group, without statistically significant difference between the two groups (P=0.447). The mortality rate in the double negative group was 3.90% (3/77), and it was 3.00% (3/100) in the double positive group (P=0.926). The duration from the onset of symptoms to the antibody test in the double negative group was significantly shorter than that in the double positive group (25.89±12.70) d vs (35.53±10.59) d, P=0.000. There was no significant difference in the remaining clinical features and laboratory tests like white blood cell count and hypersensitive C-reactive protein between the two groups (all P > 0.05).

  Conclusion  There may be immune escape in IgM and IgG double negative cases. Therefore, the clinical symptoms are relatively mild and the chest imaging is normal. However, the expression of antibody is not significantly related to the severity and prognosis of COVID-19.